The term thermistor, an acronym from thermally sensitive resistor, is accepted today as a generic name for devices made of materials, the electrical resistivity of which varies considerably with temperature. Although originally thermistors were intended for temperature measurements or for acting as temperature control elements, nowadays they have an extremely wide range of applications in various fields, for instance, in medical equipment, in the automobile industry, in communication systems. For some applications, it is desirable to achieve a maximum response of the thermistor to a temperature variation. One specific example is the use of a thermistor in the measurements of microwave power. The rate of energy flow in microwave beams is measured by allowing the beam to fall on the thermistor, the relatively small temperature rise so produced in the thermistor resulting in a relatively large change in the resistance of the thermistor, a quantity which can be determined and then serve as an indication of the microwave power. Yet, there are different uses for thermistors, where it is desirable to reduce the sensitivity of the thermistors to the temperature variation.
The thermistors are grouped according to two categories, which are defined by the arithmetic sign of the temperature coefficient of resistivity of the thermistors. This quantity, hereinafter designated as .alpha., is the fractional change in resistivity per unit change in temperature, as defined by the following equation: ##EQU1## where .rho. is the thermistor resistivity and T is the temperature. A negative value of .alpha. means that the resistivity of the thermistor decreases with increasing temperature (d.rho./dT&lt;0), a thermistor having a negative .alpha. is called NTC-thermistor, while, a PTC-thermistor is a thermistor having positive temperature coefficient of resistivity (d.rho./dT&gt;0).
NTC-thermistor materials generally follow an exponential resistivity-temperature relation: EQU .rho.=.rho..sub.0 exp(.beta./T) (II)
where .rho..sub.0 is the resistivity for T.fwdarw..infin. and .beta. is a constant characteristic of the thermistor. The relation between .alpha., the temperature coefficient of resistivity and .beta., the thermistor constant, is readily obtained by introducing the expression for .rho., given by equation (II), into the definition (I) .alpha.: ##EQU2##
The resistivity-temperature expression (II) implies that the thermistor constant .beta. is the quantity that may be directly derived from the electrical measurements of a thermistor, as a plot of ln.rho. versus 1/T should give a straight line, the slope of which equals .beta.. Accordingly, these two quantities, .alpha. and .beta., together of course with the magnitude of the resistivity of a thermistor (at any given temperature), characterize the electrical properties of the thermistor.
NTC-thermistors are usually made of semiconducting transition metal oxides, and by controlling the chemical composition and the geometrical parameters of said NTC-thermistors, it is possible to construct devices having electrical resistance in the range of about 1 to &gt;1,000,000 ohms at room temperature. NTC-thermistors are sometimes applied as thick film paste-like formulations, wherein the conductive phase, comprising a spinel type metal oxide, is surrounded by an inorganic binder, e.g., a glass binder, in an inert liquid medium used as vehicle, to achieve the desired electrical and transport properties for the formulation.
Cobalt ruthenate, Co.sub.2 RuO.sub.4, is an example of an important spinel type (AB.sub.2 O.sub.4, wherein A and B stand for metal atoms) semiconducting oxide suitable for the preparation of thick film NTC-thermistors. It is known in the art, as described in U.S. Pat. No. 5,122,302, incorporated herein by reference, that Co.sub.2 RuO.sub.4 can be synthesized by drying an aqueous dispersion of approximately stoichiometric amounts of Co.sub.3 O.sub.4 and RuO.sub.2 and then firing the dried dispersion in air at a temperature higher than 850.degree. C. Krutzsch and Kemmler-Sack, in Mat. Res. Bull., 18, p. 647 (1983) and in Mat. Res. Bull. 19, p. 1959 (1984) reported the preparation of various compositions of Co--Ru--O system, as well as transition metal containing compositions of said system, by a method involving extended sintering procedures. These articless in particular provide crystallographic and spectroscopic analysis for said systems, and are not directed to glass composites of said cobalt-ruthenate materials or thick film formulations comprising said cobalt-ruthenate materials.
There is a continuously increasing need for new thermistors and for a convenient, economical process for their preparation.
It is a purpose of the present invention to provide novel cobalt-ruthenate materials which are useful as thermistors.
It is another purpose of this invention to provide a process for preparing said thermistors which does not suffer from the prior art drawbacks, in particular a process involving relatively moderate conditions, such as improved energy consumption and short duration for the sintering stage, said process yielding substantially pure single phase materials as defined hereinafter.
It is yet a further object of the present invention to provide composites of said cobalt-ruthenate materials and glasses which are characterized by a variety of valuable electrical properties.
It is another object of the present invention to provide thick film formulations comprising said cobalt-ruthenate materials, said formulations being useful as thermistors.
Other objects of the present invention will become apparent as the description proceeds.